This invention relates to electrically conductive, temperature self-limiting heaters.
Conventional water bed heaters are unsatisfactory for a variety of reasons. These heaters usually comprise an area heater connected to an appropriate temperature controller and power source. The area heater is placed against the bottom of the water bed mattress. The heat generated by the area heater is transferred through the polyvinyl chloride mattress and into the water in the mattress.
Although such heaters have been found to be satisfactory in practice, they are not without disadvantages. For example, the hot heater tends to degrade the mattress where it is in direct contact with the mattress, thereby shortening the life of the water bed mattress. Under some circumstances, it is possible for the water bed to burst at the point of degradation. Another disadvantage arises if the mattress is inadequately filled with water. The area heater can then overheat. If the mattress bottoms out on the heater, this can result in a person sleeping on the mattress being burned. A third disadvantage is that it is inherently less efficient to heat water through a thick plastic layer than it would be to heat the water directly.
These disadvantages can be overcome with a heater which is immersible in water. Unfortunately, there is no flexible, temperature self-limiting, water-immersible heater available for water beds.
It has been suggested to directly immerse in water electrically conductive heaters such as those described in U.S. Pat. Nos. 3,823,217; 3,858,144; 3,861,029; and 3,914,363, in copending and coassigned U.S. patent application Ser. Nos. 869,786 filed by Kampe on Jan. 16, 1978; 750,149 filed by Kamath et al on Dec. 13, 1976, now abandoned; and 947,554 filed by Kamath et al on Oct. 2, 1978. Each of these four patents and each of these two patent applications are incorporated herein by this reference. Such heaters have found many commercial applications. For example, they have been disposed along pipes to either prevent fluids contained therein from freezing or to maintain the temperature of such fluids at some preselected temperature. Further, such heaters have been embedded in pavement materials to keep the surface of such pavement free from ice and snow.
Useful as such prior art heaters are, they unfortunately suffer from a number of disadvantages which make them unsuitable for applications in which they would be subjected to continuous immersion in water or other liquids. For example, such heaters can be permanently damaged by moisture or certain polymer additives which can diffuse into an immersed heater. Therefore, these prior art heaters are unsuitable for heating water beds by immersion in the water. Thus, there is a need for a flexible, temperature self-limiting, water immersible heater suitable for water beds.